Heretofore, provision of high-sensitivity silver halide photographic materials has been desired. In particular, provision of color-sensitized high-sensitivity silver halide photographic materials has especially been desired.
Color sensitization technology is extremely important and is indispensable for producing high-sensitivity photographic materials with excellent color reproducibility. A color sensitizer inherently has a function of absorbing light with a long wavelength range which is not substantially absorbed by silver halide photographic emulsions and of transmitting the energy of the absorbed light to the silver halide. Therefore, increasing the amount of light to be captured by a color sensitizer is advantageous for elevating the photographic sensitivity of a photographic material. Accordingly, attempts have heretofore been made to elevate the amount of light to be captured by a color sensitizer by increasing the amount of the color sensitizer to be added to the silver halide emulsion. However, if the amount of the color sensitizer to be added to the silver halide is greater than an optimum amount, the result is severe desensitization. This is generally called dye desensitization, which is a phenomenon resulting in desensitization in the light-sensitive range intrinsic to a silver halide substantially free from color absorption by a sensitizing dye. If dye desensitization of a photographic material is great, then the total sensitivity of the photographic material will be low even though the material may be heavily color-sensitized. In other words, decreasing dye desensitization causes a proportional elevation of the sensitivity of the light-absorbing range by a color sensitizer (namely, the color sensitivity of a color sensitizer itself). Therefore, the solution of the problem of dye desensitization is an important theme in color sensitization technology. In general, a sensitizing dye having a light-sensitivity in a longer wavelength range involves greater dye desensitization. These matters are described in T. H. James, The Theory of the Photographic Process, Forth Edition, pages 265 to 268 (published by Macmillan Publishing Co., Inc. 1977).
Methods of elevating the sensitivity of a photographic material by decreasing the dye desensitization thereof are known, as described in JP-A-47-28916, JP-A-49-46738, JP-A-54-118236 and U.S. Pat. No. 4,011,083. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) However, these proposed methods are limited to specific sensitizing dyes and the effects thereof are still unsatisfactory. The most effective means of eliminating dye desensitization presently known is a method of using bisaminostilbene compounds substituted by pyrimidine derivatives or triazine derivatives, for example, as described in JP-B-45-22189, JP-A-54-18726, JP-A-52-4822, JP-A-52-151026 and U.S. Pat. No. 2,945,762. (The term "JP-B" as used herein means an "examined Japanese patent publication".) However, the proposed compounds are only effective with a limited class of sensitizing dyes: the so-called M-band sensitizing dyes which show a gently-sloping sensitization peak in a relatively long wavelength range, such as dicarbocyanines, tricarbocyanines, rhodacyanines and merocyanines.
U.S. Pat. No. 3,695,888 discloses combination of a tricarbocyanine and an ascorbic acid to yield infrared sensitization of a photographic material; British Patent 1,255,084 discloses combination of a particular dye and an ascorbic acid to yield elevation of the minus-blue sensitivity of a photographic material; British Patent 1,064,193 discloses combination of a particular dye and an ascorbic acid to yield elevation of the sensitivity of a photographic material; and U.S. Pat. No. 3,809,561 discloses combination of a desensitizing nucleus-containing cyanine dye and a supercolor sensitizer such as an ascorbic acid.
However, the preceding technology often displays an unsatisfactory sensitizing effect of the dyes used, and even if a high sensitizing effect is attained, it often causes an increase of fog of the photographic material.
It is known that sensitizing dyes having a reduction potential of -1.25 V or more are low in a relative quantum yield of spectral sensitization as described in T. Tani et al., Journal of the Physical Chemistry, vol. 94, page 1298 (1990). It is proposed in the aforesaid The Theory of the Photographic Process, Forth Edition, pages 259-265 (1977) that super-sensitization is conducted by positive hole capture to increase the relative quantum yield of spectral sensitization. However, it is highly demanded to provide more effective supersensitizing agents.